The present invention relates to a vacuum power interrupter, and more particularly to a vacuum power interrupter wherein a chromium oxide film is formed on a surface of a stainless steel shield disposed within a vacuum vessel.
In prior art vacuum power interrupters to improve flashover voltage, known improvements are made as follows; (a) increasing the number of gaps between shields, between shield and electrical contacts, or between shield and end plates, (b) division of gaps through use of compound shields, (c) a formation of uniform gap electric filed due to an improvement of a form of a shield. However, concerning vacuum power interrupters for high voltage use, primary electron emission increases from electrical contacts or contact rods. The primary emission is accelerated by an electric field and collides with the surface of the shield.
On the other hand, shields made from stainless steel or non-magnetic material are used to suppress eddy currents. An oxide film having a thickness of 10 A is formed on the shield surface. The oxide film, such as Fe.sub.2 O.sub.3, (non-crystalline) and deposited on the shield in a vacuum of approximately 10.sup.-4 Torr with a surface analytical apparatus (Auger electron spectroscopy, or X-ray electron spectroscopy etc.). In connecting with a coefficient .delta. of emission of secondary electrons of Fe.sub.2 O.sub.3 (number of a radiative secondary electron number of a incident primary electron), its maximum value is larger than 1, as expressed by following expression .delta. max&gt;1.
Accordingly, when the primary electrons collide with the shield surface, a number of secondary electrons larger than the number of primary electrons is radiated from the shield. An insulating breakdown between shields occurs, or, between the shield and an electrical contact, or between contact rods. Therefore, with such prior art vacuum circuit breakers, it is impossible to obtain a sufficient flashover voltage.